Chem. Mater. 2001, 13, 1041-1050
1041
Synthesis of Hydrophobic Molecular Sieves by Hydrothermal Treatment with Acetic Acid Christopher W. Jones,*,†,‡ Son-Jong Hwang,† Tatsuya Okubo,§ and Mark E. Davis† Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, and Department of Chemical System Engineering, University of Tokyo, Bunkyo-ku, Tokyo, 113-8656, Japan Received October 4, 2000. Revised Manuscript Received December 9, 2000
A series of calcined borosilicate molecular sieves are treated hydrothermally with aqueous acetic acid and subsequently characterized in detail. The acid treatments are shown to expel boron from the molecular sieves, and the defects created by the boron removal are subsequently healed with silicon dissolved from other parts of the crystal. By use of this procedure, highly crystalline, hydrophobic all-silica CIT-1 and SSZ-33 (CON topology) molecular sieves are synthesized for the first time. 29Si Bloch decay (BD) and crosspolarization magic-angle spinning (CPMAS) NMR spectra indicate that the materials have few internal defects and allow for structural characterization of CON materials by 29Si MAS NMR. Seven unique tetrahedral silicon species are identified in the spectra in agreement with the published crystal structure. The effects of treatment conditions such as acid type, solution pH, and temperature are studied in detail. Conditions near the isoelectric point of silica (pH ) 0-2) are found to be efficient for the production of a variety of hydrophobic materials with few structural defects including molecular sieves of the *BEA, CON, and MWW topologies. Detailed control studies using SSZ-33 show that the nearly defect-free materials contain triangular mesopores that are clearly distinguishable in field-emission electron microscopy images. The silica that previously inhabited the mesopores is a likely source for the species required to heal the defects created by boron expulsion. In addition to healing the vacancies previously inhabited by boron with dissolved silicon, an additional aluminum or gallium source can be added to the acetic acid solution and the ions are incorporated into the framework. Many of the defects that are not filled with the Al3+ or Ga3+ species are filled with silicon during the treatment to produce a more hydrophobic metallosilicate than the one containing internal silanol defects.
Introduction Hydrophobic, pure-silica zeolites are useful materials primarily because of their organophilic character and thermal and hydrothermal stability. Microporous, puresilica molecular sieves can be synthesized hydrothermally using organic molecules, structure-directing agents (SDAs), to kinetically steer their syntheses to the desired products. However, in many cases, the presence (or absence) of tetrahedral, non-silicon atoms in the synthesis such as boron, aluminum, or zinc causes the formation of different crystalline phases or prevents the formation of a crystalline phase entirely. For example, the use of the N,N,N-trimethyl-2-adamantanammonium cation as an SDA gives SSZ-24 (AFI) when boron is included in the synthesis gel, and SSZ-13, SSZ-23, or SSZ-25 when varying amounts of aluminum are included in the synthesis.1 In the absence of any tetra* E-mail:
[email protected]. † California Institute of Technology. ‡ Current address: School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332. § University of Tokyo. (1) Zones et al. In Proceedings of the 9th International Zeolite Conference; von Ballmoos, R., et al., Eds.; Butterworth-Heinemann: Boston, MA, 1993; p 163.
hedral, non-silicon atom in the synthesis, no crystalline products are formed. Situations such as this invariably arise in zeolite synthesis and as a result, many framework topologies can only be synthesized in a narrow range of framework compositions. For example, molecular sieves of the CON topology have only been directly synthesized to date as borosilicates (SSZ-33,2 CIT-1)3 or as an aluminosilicate (SSZ-26).4 To access other framework compositions, various strategies have been employed. Dealumination of zeolites is commonly carried out to synthesize high-silica or pure-silica zeolites. Many dealumination procedures have been developed over the years including steaming,5 treatment with mineral acids6 or chelating agents,7 reaction with silicon tetrachloride8 and treatment with silicon hexafluoride.9 (2) Zones, S. I. U.S. Patent 4,963,337, 1990. (3) Lobo, R. F.; Davis, M. E. J. Am. Chem. Soc. 1995, 117, 3764. (4) Zones, S. I.; Olmstead, M. M.; Santilli, D. S. J. Am. Chem. Soc. 1992, 114, 4195. (5) McDanial, C. V.; Maher, P. K. In Molecular Sieves; Barrer, R. M., Ed.; Society of Chemical Industry: London, 1968; p 186; U.S. Patent 3,449,070, 1969. (6) Barrer, R. M.; Makki, M. B. Can. J. Chem. 1964, 42, 1481. (7) Kerr, G. T. J. Phys. Chem. 1968, 72, 2594. (8) Beyer, H. K.; Belenykaja, I. In Catalysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 203.
10.1021/cm0007961 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/23/2001
1042
Chem. Mater., Vol. 13, No. 3, 2001
Another route to both all-silica and heteroatomcontaining framework compositions is through the use of borosilicates as precursor species. Removal of boron from the framework of molecular sieves requires significantly milder conditions than does the removal of aluminum.10 Vacancies with tetrahedral coordination can then be repopulated in a subsequent step with a variety of species including silicon,10 titanium,11 and aluminum,2 among others. Recently, the zincosilicate CIT-6 (*BEA topology) was synthesized and extensively characterized.12-13 This material is unique because it can be used as a precursor to a variety of molecular sieves of the *BEA structure. The organic SDA can be removed from the micropores by solvent extraction techniques. Furthermore, zinc, like boron, can easily be removed from the molecular sieve framework under relatively mild conditions. In particular, aqueous acetic acid treatments were found to be suitable for removal of zinc from the framework of CIT-6 while simultaneously removing the organic SDA from the micropores. Under the proper conditions, zinc can completely be removed from the material, and the vacancies (defects) left behind can be healed with silicon that is presumably dissolved from other parts of the crystal. Similarly, in the development of organic-functionalized molecular sieves (OFMSs), we found that extraction of the organic SDA from the micropores of the molecular sieve using aqueous acetic acid resulted in materials that were essentially free of structural defects.14 In contrast, extraction with other solvents left a material that contained a significant number of internal defects as determined by 29Si solid-state NMR spectroscopy. In this work, we report the extension of the use of acetic acid treatment to a variety of calcined molecular sieves with different framework compositions and structures and show the generality of this methodology for preparing a broad spectrum of molecular sieve materials. Here, calcined borosilicate and pure-silica molecular sieves of different topologies are treated with acetic acid under a variety of conditions and are subsequently characterized in detail. Specific attention is paid to the role of acetic acid in this system. Experimental Section Molecular Sieve Synthesis. Borosilicate CIT-1 (CON topology) was synthesized via the published procedure using N,N,N-trimethyl (+)myrtanylammonium hydroxide as an SDA in a Teflon-lined autoclave at 160 °C.3 Borosilicate SSZ-33 (CON topology) was synthesized as described in the literature.2 The borosilicate of the MWW topology, ERB-1, was prepared using a published procedure.15 Aluminum-free, boron-containing ZSM-5 (MFI topology) was synthesized using the procedure (9) Skeels, G. W.; Breck, D. W. In Proceedings of the 6th International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: Guilford, U.K., 1984; p 87. (10) de Ruiter, R.; Pamin, K.; Kentgens, A. P. M.; Jansen, J. C.; van Bekkum, H. Zeolites 1993, 13, 611. (11) Rigutto, M. S.; de Ruiter, R.; Niederer, J. P. M.; van Bekkum, H. Stud. Surf. Sci. Catal. 1994, 84, 2245. (12) Takewaki, T.; Beck, L. W.; Davis, M. E. J. Phys Chem. B 1999, 103, 2674. (13) Takewaki, T.; Beck, L. W.; Davis, M. E. Top. Catal. 1999, 9, 35. (14) Jones, C. W.; Tsuji, K.; Davis, M. E. Microporous Mesoporous Mater. 1999, 33, 223. (15) Millini, R.; Perego, G.; Parker, W. O.; Bellussi, G.; Carluccio, L. Microporous Mater. 1995, 4, 221.
Jones et al. reported by Dwyer.16 Silicalite, the all-silica variant of ZSM5, was synthesized in the absence of alkali metal ions using a mixture of tetrapropylammonium bromide and piperizine as SDAs. Molecular sieves with the BEA topology were synthesized in a variety of forms. Borosilicate14 and pure-silica14,17 Beta molecular sieves were synthesized using tetraethylammonium fluoride as the SDA. These samples are denoted as B-Beta-F and Si-Beta-F, respectively. Pure-silica Beta was also synthesized using 4,4′-trimethylenebis(1-methyl-1benzylpiperidinium) hydroxide as an SDA using a published procedure.18 This material is denoted as Si-Beta-OH. A borosilicate Beta sample, referred to as B-Beta-OH, was also synthesized in hydroxide media following an established method.19 All materials were calcined to remove the occluded structure-directing agent. Samples were heated in an oven at approximately 2 °C/min in a nitrogen purge up to the maximum temperature of 500-675 °C at which point air was fed to the oven for 4-8 h. Acetic Acid Treatment. For experiments at temperatures above 100 °C, calcined molecular sieves were treated in Teflonlined autoclaves in an oven with rotation at ∼60 rpm for 6 days unless noted. In a typical experiment, 0.2 g of molecular sieve was added to a 45 mL autoclave filled with 25 g of water and 10 g of glacial acetic acid (pH ∼ 1.65). After 144 h of heating, the autoclave was cooled rapidly in a water bath, and the solid products were washed extensively with water and acetone and recovered by filtration. Any variations on this scheme are noted specifically in the text. Analysis. X-ray powder diffraction (XRD) patterns were collected on a Scintag XDS 2000 diffractometer using Cu-KR radiation. 11B, 27Al, and 29Si NMR spectra were recorded on a Bruker AM 300 spectrometer equipped with a cross-polarization (CP) magic-angle spinning (MAS) accessory. Samples were packed in zirconia rotors (7 mm, 29Si; 4 mm, 11B, 27Al) and spun in air. The 29Si (59.63 MHz) NMR spectra were obtained at a spinning speed of 4 kHz, with recycle delays of 10 s for Bloch decay (BD) experiments and 2-5 s for CP experiments. Contact times for CP experiments were varied from 2.5 to 4.5 ms. The 27Al (78.2 MHz) and 11B spectra (96.3 MHz) were obtained at a spinning speed of 12 kHz (using fully hydrated samples in the case of Al). Additional 29Si (99.38 MHz) BD and CP NMR experiments were carried out on a Bruker DSX500. In this case, a spinning speed of 8 kHz was used with 4 mm rotors. The DSX 500 was also used for 71Ga (152.48 MHz) BD NMR experiments with a spinning speed of 12 kHz. In all cases, experiments were referenced externally (B, aq Boric acid ) 18.8 ppm; Al, 1N Al(NO3)3; Si, tetrakismethylsilane; Ga, 1 N Ga(NO3)3). All reported 29Si NMR results are from the 300 MHz instrument unless specifically noted. Field-emission scanning electron microscopy (FE-SEM) was performed using a Hitachi S-900 at the University of Tokyo. Prior to observation, the samples were dried in an oven at 85 °C, and coated by Pt for a few seconds using magnetron sputtering (Hitachi, E-1030). The acceleration voltage for the observation was varied between 7 and 10 kV. Nitrogen adsorption isotherms were obtained at 77 K using an Omnisorp 100 sorption apparatus operating in static mode using fixed dosing. Prior to the adsorption experiment, samples were treated under vacuum at 175 °C for at least 6 h. Vapor phase water adsorption isotherms were obtained on a McBain-Bakr balance after pretreatment at 175-200 °C in situ. Nitrogen and water adsorption capacities are reported at P/Po ) 0.05 and 0.2, respectively. Elemental analyses were performed at Galbraith Laboratories, Inc., Knoxville Tennessee.
Results and Discussion Structural Characterization. All of the samples used in this study are highly crystalline, as determined (16) (17) 2365. (18) (19) 1993.
Dwyer, J.; Zhao, J. J. Mater. Chem. 1992, 2, 235. Camblor, M. A.; Corma, A.; Valencia, S. Chem. Commun. 1996, Tsuji, K.; Davis, M. E. Microporous Mater. 1997, 11, 53. Zones, S. I.; Yuen, L. T.; Toto, S. D. U.S. Patent 5,187,132,
Synthesis of Hydrophobic Molecular Sieves
Figure 1. XRD patterns of calcined Si-MFI (a), B-MFI (b), ERB-1 (c), CIT-1 (d), SSZ-33 (e), B-Beta-F (f), and Si-Beta-OH (g).
by XRD and nitrogen physisorption analysis. The XRD patterns of several of the calcined samples are shown in Figure 1, and nitrogen capacities are given in Table 1. In most cases, the acetic acid treatment results in no notable changes in the XRD patterns of the samples (XRD patterns of CIT-1 and SSZ-33 after the acid treatment are shown in Supporting Information). Exceptions are the XRD patterns of ERB-1 which show an additional peak at 2θ ) 3.5 in samples extracted under all conditions, (see Supporting Information). This extra intensity is also seen in other all-silica materials with the MWW topology.20 In contrast to the XRD results, 29Si BD NMR results show significant differences between the calcined and acetic acid treated molecular sieves in most cases. 29Si BD NMR spectra for SSZ-33 treated with acetic acid for 6 days at a variety of temperatures are shown in Figure 2. As the temperature of the treatment is increased, the amount of Q3 silicon in the sample decreases relative to Q4 silicon. In addition, there is a significant sharpening of the Q4 region, allowing crystallographically distinct T sites to be distinguished. These highly resolved NMR spectra can only result from expulsion of boron from the lattice and healing of the created framework defects with silicon. This loss of boron is confirmed by elemental analysis (Table 1). CIT-1, another borosilicate of the CON topology, behaves similarly. However, in this case, the most highly resolved NMR spectra (shown in Figure 3) appear slightly different from those of SSZ-33, likely due to the fact that SSZ-33 is an intergrowth of two polymorphs while CIT-1 is a single polymorph. High-silica samples of SSZ-33 and CIT-1 that are nearly free of defects have not been obtained previously. Reported attempts to produce materials of this nature by steaming were unsuccessful.3 The 29Si BD NMR spectra of CIT-1 treated with acetic acid show at least five readily identifiable Q4 silicon maxima at -109.35, -111.15, -111.95, -112.95, and -116.2 ppm. As there are seven crystallographically unique tetrahedral sites in the CON topology,3 further deconvolutions of the spectra are required in order to identify all the sites. Figure 4 shows the experimental 29Si BD NMR (500 MHz) spectrum of CIT-1 treated at (20) Camblor, M. A.; Corma, A.; Diaz-Cabanas, M. J.; Baerlocher, C. J. Phys. Chem. B. 1998, 102, 44.
Chem. Mater., Vol. 13, No. 3, 2001 1043
185 °C along with the simulated pattern. The pattern was successfully simulated using seven Lorentzian curves of nearly equal area, in agreement with the crystal structure of the CIT-1 (seven tetrahedral species of equal abundance). The experimental and simulated data are summarized in Table 2. Despite the fact that relatively short pulse delays are used (10 s), the simulated pattern agrees reasonably well with the published crystal structure.3 For experiments using the 300 MHz instrument, a delay time of 10 s is used, whereas the delay is varied in experiments at 500 MHz. For delay times ranging from 10 to 60 s at 500 MHz, the overall relative intensity ratio remains almost constant while the signal continuously grows as the delay time is increased. An exception to this trend is the peak at -116.49 ppm, which requires a very long delay time (∼100 s). This peak is therefore underestimated under the conditions used here. Nonetheless, the agreement between the NMR results and the published crystal structure with regard to T-site abundance is quite satisfactory under the conditions used here. More detailed structural characterization of these CIT-1 samples will be presented elsewhere.21 The 29Si BD NMR spectra of acetic acid treated ERB-1 (MWW topology) are also markedly affected by the treatment conditions. Like the molecular sieves of the CON topology, treatment at higher temperatures yields materials with NMR spectra exhibiting fewer structural defects (Q3 silicon) and increased T-site resolution in the Q4 region. The NMR spectra are illustrated in Figure 5. The spectra of the acetic acid treated materials agree quite well with the published spectrum of ITQ-1, a silicate of the same MWW topology.22 This result indicates that the present method is effective on materials that contain only 10 MR pore openings (MWW has also 12 MR pores that do not open to the exterior of the crystal) in addition to the CON materials that have 10 and 12 MR pore openings. Furthermore, the fact that the technique works on a material with a relatively high boron content (see Table 1) illustrates its versatility. While the 29Si BD NMR spectra of the acetic acid treated materials exhibit essentially no Q3 silicon, the materials are not perfectly defect-free, as evidenced by the 29Si CP NMR spectra shown in Figure 6. Expulsion of boron from the lattice and healing of defects with silicon have an effect on the sorptive properties of the molecular sieves. In most cases, a minor loss in the nitrogen adsorption capacity is observed for the acetic acid treated samples, as shown in the data listed in Table 1. The silicon that heals the defect sites in the deboronated samples must originate from some portion of the sample, as no extra silicon source is included in the procedure. This slight loss in porosity is likely due to some dissolution of the crystal. FE-SEM results do not show many significant changes in crystal morphology after treatment with acetic acid (FE-SEM images of calcined SSZ-33, CIT-1, and ERB-1 are available in Supporting Information). For the cases of SSZ-33 and ERB-1, many different crystal habits are evident, with sizes ranging from tens of nanometers to several mi(21) Hwang, S. J.; Wilkinson, A. P.; Jones, C. W. manuscript in preparation. (22) Camblor, M. A.; Corell, C.; Corma, A.; Diaz-Cabanas, M. J.; Nicolopoulos, S.; Gonzalez-Calbet, J. M.; Vallet-Regi, M. Chem. Mater. 1996, 8, 2415.
1044
Chem. Mater., Vol. 13, No. 3, 2001
Jones et al.
Table 1. Synthetic and Physical Parameters of Samples sample
topologya
treatment
CIT-1 CIT-1 CIT-1 SSZ-33 SSZ-33 SSZ-33 SSZ-33 SSZ-33 SSZ-33 SSZ-33 ERB-1 ERB-1 ERB-1 ERB-1 ERB-1 ERB-1 Si-MFI Si-MFI B-MFI B-MFI B-MFI Si-Beta-OH Si-Beta-OH Si-Beta-OH B-Beta-F B-Beta-F B-Beta-F
CON CON CON CON CON CON CON CON CON CON MWW MWW MWW MWW MWW MWW MFI MFI MFI MFI MFI *BEA *BEA *BEA *BEA *BEA *BEA
none aq acetic acid aq acetic acid none aq HCl aq acetic acid aq acetic acid aq acetic acid aq acetic acid aq acetic acid none aq HCl aq acetic acid aq acetic acid aq acetic acid aq acetic acid none aq acetic acid none aq acetic acid aq acetic acid none aq acetic acid aq acetic acid none aq acetic acid aq acetic acid
pHb
temp. (C)
time
1.65 1.65
160 185
6 days 6 days
2.00 1.65 1.65 1.65 1.65 1.65
25 110 135 160 185 135
5 hours 6 days 6 days 6 days 6 days 14 days
2.00 1.65 1.65 1.65 1.65
25 135 160 185 185
5 hours 6 days 6 days 6 days 6 days
1.65
185
6 days
1.65 1.65
135 185
6 days 6 days
1.65 1.65
135 160
6 days 6 days
1.65 1.65
135 185
6 days 6 days
Si/Bc
N2 ads (mL/g)d
H2O ads. (mL/g)e
20.9 >2000 NDf 19.4 ND 156.5 ND ND 494.0 ND 10.9 36.5 75.0 ND ND ND
0.225 0.244 0.209 0.181 0.173 0.186 0.186 0.184 0.175 0.173 0.165 0.159 0.170 0.164 0.146 0.141 0.144 0.153 0.138 0.143 0.140 0.285 0.248 0.226 0.231 0.239 0.099g
0.034 0.013 0.006 0.063 0.055 0.020 0.017 0.015 0.006 0.009 0.045 0.087 0.036 0.013 0.007 0.011 0.011 0.013 0.019 0.013 0.009 0.045 0.047 0.030 0.039 0.039 0.011
51.4 >900 >400
29.2 >350 >1000
a International Zeolite Association structure code. b Initial solution pH. c Si/B ratio as determined by elemental analysis. d As determined by nitrogen adsorption, 77 °C P/Po ) 0.05. e Water adsorption on a McBain-Baker balance, 25 °C, P/Po ) 0.2. f ND ) not determined. g Some structural collapse.
Figure 2. 29Si BD NMR spectra of calcined SSZ-33 (a), calcined SSZ-33 that was deboronated by treatment with 0.01 N HCl for several hours (b) and calcined SSZ-33 treated with acetic acid for 6 days at 110 (c), 135 (d), 160 (e), and 185 °C (f).
crons. After treatment at various temperatures with aqueous acetic acid, the FE-SEM images of the materials are virtually unchanged with regard to particle morphology. However, when the materials are treated in a manner that results in solids where nearly all of their defects are healed as judged by 29Si BD NMR, notable changes in surface features are apparent in
Figure 3. 29Si BD NMR spectra of calcined CIT-1 (a) and CIT-1 treated with acetic acid for 6 days at 160 (b) and 185 °C (c).
some of the FE-SEM images. Figure 7 shows the FESEM images of SSZ-33 treated at 110 °C for 6 days (defects not healed), SSZ-33 treated at 135 °C for 14 days and 185 °C for 6 days (defects healed), and calcined SSZ-33 for comparison. For the SSZ-33 samples where the defects have been healed, triangular mesopores are evident in the samples. The number of mesopores appears to be higher on samples treated at the higher temperature. In Figure 7d (185 °C treatment), very small particles are visible on the surface of the crystals
Synthesis of Hydrophobic Molecular Sieves
Chem. Mater., Vol. 13, No. 3, 2001 1045
Figure 4. Simulated (bottom) and experimental (top) 500 MHz 29Si BD NMR spectra of CIT-1 treated at 185 °C. Table 2. Assignment of 29Si BD NMR Lines of CIT-1 Extracted at 185 °C 300,a db
500,c db
300,a Id
500,c Id
300,a fwhme
500c
-109.49 -109.93 -111.16 -111.5 -112.2 -113.2 -116.45
-109.61 -109.98 -111.03 -111.53 -112.15 -113.22 -116.49
1.1 1.1 1.3 1.3 0.9 0.9 0.8
1.1 1.0 1.2 1.0 0.7 1.0 0.9
0.635 0.948 0.736 0.441 0.489 0.389 0.482
0.515 0.870 0.600 0.358 0.302 0.338 0.311
Figure 6. 29Si CPMAS NMR spectra of CIT-1 (a), SSZ-33 (b), and ERB-1 (c) with a 2.5 ms contact time.
a In megahertz with a 10 s pulse delay. b Chemical shift, in parts per million. c In megahertz with a 60 s pulse delay. d Intensity, (normalized; crystal structure predicts all should have intensity ) 1). e Full width at half-maximum.
Figure 5. 29Si BD NMR spectra of calcined ERB-1 (a), calcined ERB-1 that was deboronated by treatment with 0.01 N HCl for several hours (b) and calcined ERB-1 treated with acetic acid for 6 days at 135 (c), 160 (d), and 185 °C (e).
that are not present in the other images. These particles may result from dissolution of the small spherical particles in the sample. Another possibility is that they originate from silica that was previously part of the large crystals in the locations of the mesopores. The presence of the mesopores is important for two reasons. First, it indicates that the silica required to
Figure 7. FE-SEM images of calcined SSZ-33 (a) and calcined SSZ-33 treated at 110 °C for 6 days (b), treated at 135 °C for 14 days (c) and treated at 185 for 6 days (d).
heal the defect sites of the materials may come from the crystal itself, rather than from the smaller particles present via an Ostwald’s ripening mechanism. This observation, however, does not rule out the possibility that the smaller crystals are dissolved in order to provide silica that heals defects in the larger ones, although the relative abundances of the small and large particles do not appear to change after treatment with acetic acid. The second important implication of the presence of mesopores concerns the transport of soluble
1046
Chem. Mater., Vol. 13, No. 3, 2001
Figure 8. 29Si BD NMR spectra of calcined B-MFI (a), calcined B-MFI treated with acetic acid at 185 °C for 6 h (b), calcined Si-MFI (c), and calcined Si-MFI treated with acetic acid at 185 °C for 6 h (d).
Jones et al.
Figure 9. 29Si BD NMR spectra of calcined B-Beta-F (a), calcined B-Beta-F treated at 135 °C for 6 days (b), calcined B-Beta-F treated at 185 °C for 6 days (c), and calcined, defect-free all-silica Beta synthesized in fluoride media (d).
silica species, as the presence of large mesopores would greatly enhance the movement of these species through the pores to the defect sites that they must heal. Note that although mesopores are detected clearly by FESEM, significant differences in the nitrogen physisorption isotherms of materials with and without mesopores were not detected due to the low cutoff pressure of the experiments (200 Torr). Another consequence of the expulsion of boron and healing the defects with silicon is an increase in hydrophobicity. The data in Table 1 illustrate that there is a significant decline in the water adsorption capacities of the materials after treatment with acetic acid, indicating increased hydrophobicity resulting from replacement of boron with silicon in the lattice. The hydrophobicity as measured by the water adsorption uptake correlates nicely with the population of Q3 silicon as determined by 29Si BD NMR (less Q3 silicon ) lower water adsorption). Furthermore, the hydrophobicities of the materials compare well with those of calcined, defect-free materials such as calcined, pure-silica Beta synthesized in fluoride media ( 7, 2 < pH < 7, and pH < 2.32-33 At pH’s in excess of 7, condensation occurs by nucleophilic attack of OH-. In the pH range of 2-7, the same mechanism is thought to occur, with the condensation rate proportional to the concentration of OH-. In contrast, below a pH of 2, the rate is proportional to the concentration of H+ (in the absence of F-). It is under these conditions, where the solubility of silica is low (0 < pH < 2), that acid treatment is shown here to be very effective at producing nearly defect-free, pure silicates from borosilcates or zincosilicates12 without an external silicon source. At
(31) Kornatowski, J.; Baur, W. H.; Pieper, G.; Rozwadowski, M.; Schmitz, W.; Cichowlas, A. J. Chem. Soc., Faraday Trans. 1992, 88, 1339 and references therein.
(32) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1990; p 103. (33) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
Figure 11. 29Si NMR spectra of calcined, deboronated SSZ33 (a), calcined SSZ-33 treated with aqueous Al(NO3)3-9H2O and 29Si BD NMR (b), calcined SSZ-33 treated with aqueous Al(NO3)3-9H2O and 29Si CP NMR (c), and calcined SSZ-33 treated with Al(NO3)3-9H2O-containing acetic acid solution at 160 °C for 6 days and 29Si BD NMR (d).
1050
Chem. Mater., Vol. 13, No. 3, 2001
these conditions, the dissolution of the molecular sieve is slow enough to prevent significant loss of microstructure while at the same time allowing sufficient dissolution of silica to provide soluble species for healing of the defects. The use the aqueous acetic acid treatment on molecular sieves was first used to extract TEAF from the pores of OFMSs12,34 and subsequently applied to CIT-6 for a similar purpose.12 In both cases, it was found that extraction with TEAF produced completely porous materials that were nearly free of structural defects. In contrast, extraction of OFMSs with aqueous pyridine (pH > 7) or a 50/50 mixture of pyridine and 1 N HCl (pH ) 5.65), as well as extraction of CIT-6 with 1 N NH4NO3 (pH ) 4.8)12-13,25 gave porous materials that contained internal structural defects.12 In light of the results presented here, it is clear that pH is an overriding factor that controls the presence or absence of structural defects during extraction of SDAs or in the treatment of calcined molecular sieves. By use of the treatment in acidic media (0 < pH < 2), materials that are nearly free of defects can be prepared from Si-Beta or OFMSs made from TEAF,14,25 Si-Beta made using TEAOH,35 and CIT-6 (zincosilicate Beta) prepared with TEAOH.12,25 Similarly, we show here that materials essentially free of defects can be synthesized from calcined SSZ-33, B-Beta, CIT-1, and ERB-1 using similar treatments. An equally important factor to pH is temperature. Materials that are in essence free of defects can be prepared with no loss of porosity relative to the calcined materials during SDA extraction in aqueous acetic acid at temperatures below 100 °C when the molecular sieves contain F- (OFMSs and Si-Beta made from TEAF). At elevated temperatures, some porosity is lost (likely due to structural degradation by F-). In contrast, CIT-6 (which is synthesized in OH- media) treated with acetic acid under 100 °C leaves a porous all-silica material with many defects. Treatment at temperatures above 100 °C is required to heal the defects. Hence, in the absence of F-, the healing of defects by silicon requires (34) Jones, C. W.; Tsuji, K.; Davis, M. E. Nature 1998, 393, 52. (35) Takewaki, T.; Hwang, S. J.; Yamashita, H.; Davis, M. E. Microporous Mesoporous Mater. 1999, 32, 265.
Jones et al.
higher temperatures. A similar trend is observed from treatment of calcined materials. In this case, moderate temperatures (135 °C) are required to expel boron from calcined B-Beta-F and heal defects, whereas higher temperatures lead to significant structural degradation, possibly due to traces of residual F-. For materials that contain no F-, such as ERB-1, SSZ-33, or CIT-1 (molecular sieves that were synthesized in hydroxide media), higher temperatures give better results (160-185 °C) with limited loss in porosity due to structural degradation. Hence, as observed during SDA extraction from as-made molecular sieves, calcined samples synthesized in the presence of F- require lower temperatures for effective healing of defects with silicon. Conclusions Hydrothermal treatment of calcined borosilicates and silicates with aqueous acetic acid near the isoelectric point of silica (pH ) 0-2) effectively produces hydrophobic silicates with few structural defects. Defect sites are healed via dissolution of silica within the crystal, creating mesopores, followed by migration of the solublized silica throughout the material to heal internal defects. Solution pH and temperature are found to be important variables in controlling the properties of the resulting materials, with higher temperatures resulting in samples with fewer defects but slightly less porosity. Mineral acids can be substituted for organic acids. By use of this technique, all-silica CIT-1 and SSZ-33 are synthesized for the first time. CIT-1 with very few internal defects is characterized by 29Si MAS NMR spectroscopy and reveals seven unique silicon atoms, in agreement with the published crystal structure. Acknowledgment. This work was partially supported by Chevron. C.W.J. thanks Akzo Nobel and BP Amoco for additional financial support. Supporting Information Available: XRD patterns and FE-SEM images of CIT-1, SSZ-33, ERB-1, and so on; 29Si BD NMR spectra of Si-Beta-OH and B-Beta-OH; nitrogen physisorption isotherms for B-MFI and Si-MFI (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0007961
